Communications system

ABSTRACT

The present invention consists of an information structure conceived for the transport of data in digital form from a transmitting element to a receiver. This structure calls for fields for transport of the data and heading information fields termed “overhead” which improve transmission reliability. This structure enables support of digital interconnections in an element of a transport network capable of switching various traffic types such as CBRx (for example STM-N e OC-N), VC-N, STS-N or ODUk. The structure also enables identification of the frame beginning, verification of the integrity and correctness of the switching, support of protection switching, and transport of quality and timing information associated with the switched entities.

The present invention relates to a system for communication of data ingeneral and a structure or frame of information for the transport ofdata including types SDH, SONET and OTN and overhead information in anetwork element in a telecommunications system. A method and apparatusare also proposed.

Equipment inserted in a transport network realizing traffic dataswitching requires a means of transporting internally therein thetraffic input interface data to the switching structure and from theswitching structure to the traffic output interface.

One way of transferring the traffic data inside the network elements isto map said data with other information in a dedicated informationstructure suitable for transporting the interconnected entity.

Depending on the type of traffic, different information structures areneeded. Handling different types of traffic in the same equipment istherefore complex.

The general purpose of the present invention is to remedy the abovementioned shortcomings by making available an information structurewhich would allow by itself collection and transportation of the dataand information of different types of traffic and added informationallowing improvement of transmission reliability.

In particular the innovative structure in accordance with the presentinvention is for example able to transport alternatively the followingtypes of traffic:

-   -   Synchronous Digital Hierarchy (SDH) VC3, VC-4, VC-4-nc, where        n=4, 16, 64 or 256 as defined in ITU-T Recommendation G.707.    -   SONET STS-1s, STS-nc, where n=3, 12, 48, 192, 768 as defined in        Telecordia GR-253.    -   Optical Transport Network Hierarchy (OTN) ODUk, where k=1, 2 or        3 as defined in ITU-T Recommendation G.709.    -   Constant Bit Rate Signals (CBR) CBRx, where x=2G5, 10G, 40G as        defined in ITU-T Recommendation G.709 and in particular:        -   a) CBR2G5 is a constant bit rate signal of 2.488.320            kbit/s±20 ppm (for example, SDH STM-16 or SONET OC-48),        -   b) CBR10G is a constant bit rate signal of 9.953.280            kbit/s±20 ppm (for example, SDH STM-64 or SONET OC-192), and        -   c) CBR40G is a constant bit rate signal of 39.813.120            kbit/s±20 ppm (for example, SDH STM-256 or SONET OC-768).

Given its flexibility this type of structure is usable not only within atelecommunications system network element capable of switching only oneof SDH, SONET or OTN data but also in a network element whosecommunication platform allows simultaneous permutation of severaltraffic types.

In view of this, it was sought to provide in accordance with the presentinvention a frame structure designed to support digital interconnectionsbetween a transmitting element and a receiving element for the alternatetransport of different types of traffic between them and comprising atleast an overhead section and a data section sized to allow mappingtherein of the overhead information and the characteristic data of eachalternatively transported traffic type.

In addition it was sought to realize a method of information transportfrom an input interface to an output interface of a network elementcapable of switching different traffic types and including the steps offorming an information transport frame comprising a plurality of fixedsize sequential frames each including at least one overhead section, adata-stuffing section and a data section with the data-stuffing sectionand the data section being sized to be able to contain together at leastthe data of the traffic type which it requires most among thoseforeseen. Upon reception on the input interface of data of a traffictype, mapping in the frame of said data stuffing with said data all thedata section and continuing in the data-stuffing section and if thetraffic type requires less space that that provided in the frame,stuffing the surplus space with stuffing bytes to hold the size of theframe unchanged with changes in the type of traffic transported. Anapparatus in accordance with the method and including the abovementioned structure is also proposed.

To clarify the explanation of the innovative principles of the presentinvention and its advantages compared with the prior art there isdescribed below with the aid of the annexed drawings a possibleembodiment thereof by way of non-limiting example applying saidprinciples. In the drawings:

FIG. 1 shows the whole structure of the entire frame structure inaccordance with the present invention,

FIG. 2 shows the mapping justification mechanism of an ODU1 in thestructure in accordance with the present invention,

FIG. 3 shows the synchronization mechanism between the transmitter andthe receiver by alignment word,

FIG. 4 shows the information transport mechanism on the quality of theentity switched in the overhead sections, and

FIG. 5 shows the transport mechanism for information in the overheadsections allowing verification of the accuracy and quality of theswitching.

With reference to the Figures, FIG. 1 shows the whole frame structurerealized in accordance with the present invention and allowingalternative transport of different traffic types while keeping all thecharacteristics and furthermore transporting specific additionaloverhead information so as to allow traffic transport from the inputinterface to the output interface of a generic network element usingsaid structure.

As may be seen in the Figure, the frame structure in accordance with thepresent invention is organized in 4 sections, to wit, a pOTN overheadsection, a data stuffing section, a pSOH overhead section and a datasection.

The frame format of the present invention is structured according to apreferred embodiment in 9 rows and 4416 time slot columns for a total of39744 bytes and a period of 125 μs (with bit rate at 2.543616 Gb/s).Columns 1 to 12 (i.e. pOTN overhead section) always contain the headingfor work or overhead information transport. Column 13 to 96 (i.e.stuffing and data section) contain bytes of data/stuffing in the case ofODUk or CBRx data transport but otherwise they are completely stuffedwith predetermined fixed bytes (i.e. stuffing bytes) which do nottransport useful information. Columns 97 to 240 (i.e. the pSOH overheadsection) contain data bytes in case of ODUk or CBRx data transport butotherwise in case of VC-n or STS-n transport they are dedicated foroverhead information transport. Columns 241 to 4416 (i.e. the datasection) always contain data bytes.

It should be noted that even in the case of data transport a section cancontain some stuffing bytes. This is due to the difference in the sizesof the sections in accordance with the present invention, deputed todata transport, and to the different quantity of data to be transporteddepending on the type of interconnection entity handled. In particularthe sizes of the frame sections deputed to data transport are sized forthe transport of the number of data bytes of the worst case (in terms ofquantity of data bytes which are to be transported) i.e. for ODU3.Depending on the type, each interconnection entity keeps a definitenumber of bytes placed in definite positions in the frame.

This way the interconnection entities ODU2 and CBR10G are distributed in4 frames containing overhead information in the pOTN overhead sectionand data/stuffing bytes in all the other sections of the frame.Interconnection entities VC-4-64c/STS-192c are distributed in 4 framescontaining overhead information in the pOTN and pSOH overhead sectionsand data/stuffing bytes in all the other sections of the frame.Interconnection entities ODU3 and CBR40G are distributed in 16 framescontaining overhead information in the pOTN overhead sections anddata/stuffing bytes in all the other sections of the frame.Interconnection entities VC-4-256c/STS-768c are distributed in 16 framescontaining overhead information in the pOTN and pSOH overhead sectionsand data/stuffing bytes in all the other sections of the frame.

Each frame in accordance with the present invention can transport up to48 VC-3/STS-1 or 16 VC-4/STS-3c or 4 VC-4-4c/STS-12c or 1VC-4-16c/STS-48c or a mixture thereof. In these cases the overheadsections (i.e. the pOTN and pSOH sections) are deputed to overheadinformation transport, the data-stuffing section is stuffed withstuffing bytes while the data section is used for transport of theentity with the following stuffing rules:

-   -   a VC-3/STS-1 is transported by 783 bytes (i.e. 87 columns/time        slots);    -   a VC-4/STS-3c is transported by 2349 bytes (i.e. 261 time        columns/slots);    -   a VC-4-4c/STS-12c is transported by 9396 bytes (i.e. 1044 time        columns/slots);    -   a VC-4-16c/STS-48c is transported by 37584 bytes (i.e. 4176 time        columns/slots).

It should be noted that mapping of VC-n/STS-n in the frame is realizedassuming a preadaptation of the entities to the system clock of thenetwork elements before mapping in the structure in question. This isdue to the fact that SONET and SDH are hierarchies of synchronoustransport which require synchronization of all network elements. Theseentities accordingly use a fixed number of bytes in the frame and alwaysin the same position.

Concerning transport of ODUk, adaptation to the system clock of thenetwork elements is realized only to allow adaptation of the ODUk tointernal frame frequency. This is necessary to allow simultaneousinterconnection both of ODUk and VC-n/STS-n within the same structure.In case of a network element assigned exclusively to ODUk switching,because OTN is not a synchronous transport hierarchy, adaptation to asingle system synchronism would not be necessary. Adaptation is realizedduring mapping of the interconnected entity within the frame inquestion. After interconnection the clock of the entity is recovered andthe OTN signal is generated with its original timing. This means thatthe number of bytes of ODUk data transported by the structure inaccordance with the present invention can vary and consequently also theposition of the ODUk in it.

To support this adaptation a justification mechanism is supported in thedata stuffing section of the frame.

Before mapping in the internal frame, 8 bytes (i.e. 6 for an alignmentword, 1 for control and 1 for parity verification) are added for eachblock of 3824 bytes of the ODUk.

In case of ODU1 transport, the structure in accordance with the presentinvention calls for a justification mechanism capable of mapping in arange of [39122-39128] bytes of data per frame and capable of supportingany difference admitted by the standards between the frequency of theentering traffic signal and the system clock of the network elements.

FIG. 2 shows the justification mechanism for the mapping of an ODU1 inthe frame in question.

As may be seen in the Figure, the stuffing-data section is partly filledwith the stuffing bytes (i.e. empty cells in the FIG) and data bytes(i.e. D cells in the FIG). In addition, up to 6 bytes are expected to bestuffed with stuffing bytes or data bytes (i.e. cells X, Y and Z in theFIG). The content of the latter depends on the difference of frequencybetween ODU1 and system frequency and is controlled by a codifiedprotocol respectively in the control cells A, B and C. The value of theprotocol is copied three times in the same frame (i.e. 3 bytes A, B, Care present in the same frame) to allow correct interpretation throughmajority voting.

The same mechanism is used for mapping of ODU2 and ODU3. Naturally,because of the different number of bytes to be transported per frame inthe case of ODU2 and ODU3 the justification mechanism control bytes areplaced in different cells in the stuffing-data section with respect tothe ODU1 transport.

The CBRx signals are transported in the structure through apreadaptation of the CBRx bit rate to the related ODUk (i.e. CBR2G5 ->ODU1, CBR10G -> ODU2 and CBR40G -> ODU3). This preadaptation is achievedby adding stuff bytes and a frame alignment signal is added to identifythe position of the “pseudo ODU” (i.e. the adapted CBRx) within theframe.

As mentioned above, the structure described here calls for a framesection termed overhead dedicated to housing the overhead information.

First of all a frame alignment word is inserted at the source to allowidentification of the beginning of each frame at the destination.

This alignment word must have a rather resistant code so as to reduce toa minimum the chance that a similar sequence might be found in the restof the frame and cause false alignments.

For example, as shown in FIG. 3, an alignment word made up of 8 bytesand containing the hexadecimal value A1 in the first 4 bytes and A2 inthe second four meets the requirements of resistance and can be locatedin the first 8 bytes of row 1 of the pOTN overhead section.

In addition the innovative structure also calls for a multiframealignment signal, e.g. a counter [0-255] and by means of this counter[0-255] it is possible to identify intervals up to 32 ms.

Additional information on the quality of the traffic received from theinput interface will be transmitted to the switching structure torealize network protection. The most common network protection diagramscall for traffic data duplication at some point along the path,transmission along two different sub-paths (i.e. work and protection)and at the end point of the protected subnetwork selection of one of thetwo signals on the basis of a quality criterion. If the selection ismade at the switching structure level and if the quality is monitored atthe input traffic interface level this information will be forwarded tothe communication structure. Forwarding will be the fastest possible toallow the structure immediate selection of the intact traffic. Onesolution consists of associating the quality information with theassociated entity interconnected in the same means used to transmit thetraffic data from the input interface to the switching structure. Thisway the switching structure can monitor the work and protection qualityinformation and consequently select the traffic which must be routedalong the network.

The structure of the present invention calls for a frame sectiondedicated to housing the quality information of each interconnectedentity transported in it.

For example, as shown in FIG. 4, in the overhead section pOTN in adedicated byte Q the quality of the ODUk or the STM-n/OC-n transportedwill be codified while in the overhead section pSOH 48 Qn bytes will bededicated to the coding of the quality of the VC-n/STS-n transported.

A possible coding is the following:

-   -   If from the input interface a serious defect on the switched        entity is found the hexadecimal value 02 will be inserted in the        quality byte (Q) associated with the entity in question.

If from the input interface an error rate on the switched entity isfound such that it does not completely discredit the quality thereof butonly indicates a deterioration, the hexadecimal value 01 will beencoded.

-   -   If no defect and deterioration on the switched entity from the        input interface is found the hexadecimal value 00 will be        encoded.

In addition, since some of the network protection schemes (for exampleMSP and MS-Spring as defined in ITU-T G.841 for SDH) call for astandardized Automatic Protection Switching (APS) protocol to coordinatethe behavior of the two switching nodes through dedicated bytes in theoverhead (OH) of the traffic signals (for example, bytes K1 and K2 inthe multiplex section of an STM-n signal, APS bytes in the overhead ofan ODUk) the frame in accordance with the present invention isstructured to transport for each type of interconnected entity the APScommands detected at the traffic input interface to the switchingstructure. In the switching structure-traffic interface direction theframe also transports the protection state.

To allow fast implementation of the 1:N protection diagrams (for exampleMPS 1:N as defined in ITU-T G.841 for SDH) between the trafficinterfaces with different configurations the structure dedicates asection of the overhead to the transport of a protocol between trafficinterfaces and the switching structure. Upon reception of a commandthrough the APS protocol or detection of poor quality of aninterconnected entity involved in a 1:N protection, the switchingstructure instructs the protection interface to take on theconfiguration of the faulty one. In the opposite direction theprotection interface confirms the adoption of the configuration request.

The frame in accordance with the present invention is also structured tosupport protection switching methods which perform switching actions atboth ends of the protection entity (for example connection, path) evenin case of one-way failure (i.e. dual ended protections).

Another mechanism can be provided for monitoring the correctness of thetraffic routing in the network element. This is necessary to monitor thequality (i.e. the network element does not cause traffic deterioration)and connection correctness (the network element ensures connectionbetween each pair of input & output ports without introducing erroneousconnections). This mechanism must allow end to end monitoring of thetraffic data path in the network element (i.e. traffic input & outputinterfaces).

In addition, assuming a distributed network element (i.e. traffic andswitching structure interfaces in different sites) or a switchingstructure organized in different stages (for example, a Clos switchingstructure) there will be realized the monitoring of each path section ofthe entity interconnected in the network element (i.e. from the trafficinput interface to the switching structure, between each switchingstructure stage, and from the switching structure to the traffic outputinterface.

A way of obtaining these types of control is to insert in the innerframe a frame source identifier (Path Trace) and a monitoring error codecalculated after scrambling (for example the Bit Interleave Parity (BIP)defined by ITU-T G.707).

At the terminal point the Path Trace is extracted and compared with theexpected one. Detection of an inequality identifies a connection error.

Parity calculation is done at the destination before de-scrambling andthen compared with the BIP contained in the next frame extracted afterde-scrambling.

A frame section is dedicated to implementing Path Trace and Parity Checkof the end to end path of the interconnected entity in the networkelement.

Another frame section is dedicated to implementing the same checks for apath segment from end to end of the interconnected entity in the networkelement.

For example, as shown in FIG. 5, it is possible to dedicate in the pOTNoverhead section three bytes (for example, A B C) for control of thewhole input output traffic interface path of the structure in case ofswitching of CBRx signals or ODUk entities. Two bytes (for example A andB) will be dedicated to containing a univocal (i.e. unique) structuregenerator identifier while a third (for example C) will contain thegenerated structure parity. Again in the same case, another three bytes(for example E and F and G) can be dedicated to making the same type ofcontrols but on individual path sections from the structure to theinside of the network element.

As protection switching realized by the switching structure could causedetection of a parity or path trace error at the end point located onthe traffic output interface, a section of the structure is dedicated tothe transport of an activation and deactivation protocol for detectionof the error.

The frame is also designed to transport time information.

The system clock of a network element inserted in a synchronoustransport network is generally hooked to a synchronization sourceselected from a set of possible sources including the traffic interfaces(for example STM-n signals in SDH).

By means of a pointer mechanism, the frame allows the transport,together with the traffic data to be switched, of up to two timingsignals from the traffic interface which can even be hundreds of metersdistant up to the network element core where selection of the timingsource is done.

The position of the timing signal edge in the data frame in accordancewith the present invention is identified by a particular pointer valuein the dedicated overhead bytes of the frame. The pointer value iscopied n times in the same frame overhead to allow correctinterpretation at the destination through majority voting. In case of afaulty reference timing signal a particular value will be inserted inthis field to indicate that the pointer value is not valid.

The synchronization quality information (also called SSM-Timing Marker)of the timing signal transported by the frame through the pointermechanism is also contained in a dedicated section of the inner frame.

To avoid protection switching because of an error in informationtransmission by means of the inner frame from the traffic inputinterface to the switching structure, a byte dedicated to parity controlis provided in the structure in accordance with the present invention toverify the quality of the overhead section transporting the qualityinformation, the APS command and the dual ended data of theinterconnected entities and the timing signal synchronization qualityinformation. In other words, a bit of the pOTN or pSOH sectioninterpreted wrongly by the switch because of a momentary and localdeterioration could cause erroneous protection tripping. For this reasona finer control is inserted in the fields bearing a certain type ofinformation.

To facilitate alarm correlations in the network element the framesupports information exchange between the traffic input and outputinterfaces. If at the output traffic level a basic error detectioncommunication is detected in a dedicated byte of the frame in question,any possible correlated defect detected on the interconnected entity issuppressed.

It is now clear that the predetermined purposes have been achieved bymaking available a flexible frame structure allowing transport of allinformation and data necessary for transport of various types of trafficin the network element. In particular the frame allows CBRx traffictransport without distinction (for example STM-N and OC-N), VC-N, STS-Nand ODUk.

In accordance with the present invention it is thus possible to have aninformation structure consisting of traffic data transport fields andheading information fields organized in frames which are repeated withrelatively high frequency (advantageously every 125 μs) and which can beused transparently to support digital interconnections in a element of atransport network capable of switching Optical Data Units (ODU), orsynchronous transport modules (Synchronous Digital Hierarchy) STM-N,SONET synchronous transport signals STS-N derived from OD-N opticalcarriers, or virtual containers SDH VC-3, VC-4 and/or VC-4-nc, wheren=4, 16, 64, 256, and/or synchronous transport systems SONET STS1s,STS-nc, where n=3, 12, 48, 192, or 768 as defined in Telecordia GR253.

In addition, in the structure are supplied means for identification ofthe frame start, verification of the integrity and correctness of theswitching, protection switching and transporting the quality and timinginformation associated with the switched entities.

The above description of an embodiment applying the innovativeprinciples of the present invention is given by way of non-limitingexample of said principles within the scope of the exclusive rightclaimed here. For example, additions to or variants of the structure canbe considered to expand or integrate the information transported and theperformance of the system. An apparatus in accordance with the methodand structure proposed is readily realisable to those skilled in theart.

1. A method of transporting traffic information from an input interfaceto an output interface of a network element capable of switchingdifferent types of traffic information, the method comprising the stepsof: forming an information transport frame structure comprising aplurality of fixed size sequential frames with each frame comprising atleast one overhead section, one data stuffing section, and a datasection, the data stuffing section and the data section being sized forcontaining together at least the traffic type which requires mostcapacity among those expected and, upon reception on the input interfaceof data of a traffic type, mapping in the frame said data filling withsaid data all the data section, and continuing in the data stuffingsection and, if the traffic type requires less space than that arrangedin the frame, filling extra space with stuffing bytes to maintain theframe size with a change in transported traffic type, configuring thestructure with fixed dimensions and being repeated at regular intervals,and the traffic type transported being one of optical data unit-indexunit k (ODUk), synchronous transport signals (STS-n), synchronousdigital hierarchy (SDH) virtual containers (VC-n), or constant bit ratesignals (CBRx).
 2. An apparatus, comprising: at least one inputinterface; at least one output interface; and a transmitter fortransmitting data between the interfaces in a frame structure forsupporting digital interconnections between a transmitting element and areceiving element for alternatively transporting therebetween differenttypes of alternative traffic including traffic comprised in optical dataunit-index unit k (ODUk) traffic and traffic not comprised in ODUktraffic, the structure having at least one overhead section sized toallow mapping therein of overhead information, and a data section sizedto allow mapping therein of characteristic data of each type ofalternative traffic transported, the structure having fixed dimensionsand being repeated at regular intervals, and wherein the traffic typetransported is one of ODUk, synchronous transport signals (STS-n),synchronous digital hierarchy (SDH) virtual containers (VC-n) orconstant bit rate signals (CBRx).